I had the Zu Mobius for awhile. The difference it made was pretty big in my opinion. It brings to the Sennheiser sound a vibrant energy that makes it seem like you are hearing more detail (maybe you are!). All the notes are sharper, crisper and more forward. But I found this added brightness to be fatiguing over long listening sessions. This wasn't Grado bright, mind you, but if I wanted bright, I could just listen to Grados. I wanted the Sennheiser trademark dark, laid back characteristic back, and so I went with the Cardas cable. It was just what I was looking for. Smooth and laid back. Different strokes for different folks.
I've been curious about the Blue Dragon II as well but it wasn't available when I went with the Cardas. And the Blue Dragon I was sold out at the time. I'm interested in your impressions as well.